Relative Susceptibility of Four Species of Mosquitoes to Insecticide Residues

Abstract

Strains of mosquitoes resistant to chlorinated hydrocarbon insecticides have been discovered in many areas where intensive control programs have been conducted (Brown, 1958). In most instances organophosphorus insecticides have satisfactorily re-established control. Some vigor tolerance to the organophosphorus compounds by the resistant mosquitoes has been noted, but to date no cross-resistance has been reported. Often a toxicant effective against one species may be less effective against another. Residual tests were made with four species of mosquitoes to determine their relative susceptibility to Bayer 29493 (Baytex; O,O-dimethyl 0-[4- (methylthio) m-tolyl] phosphorothioate), DDT, and malathion. The mosquitoes were reared in laboratory colonies and included a strain of Aedes aegypti (L.) that had no previous exposure to insecticides, a strain of Anopheles quadrimaculatus Say known to be resistant to dieldrin, but not to DDT, a strain of Aedes taeniorhynchus (Wied.) that was resistant to DDT (Davis et al., 1959), and a strain of Anopheles albimanus Wied. that exhibited resistance to DDT and dieldrin when collected in El Salvador, but has not been subjected recently to insecticide pressure. The first three colonies were reared at Orlando, Florida; the albimanus were generously provided by L. E. Roseboom and J. H. Hobbs from a colony maintained at Johns Hopkins University. The three insecticides were tested against one- to two-day-old adults of the four species in duplicate tests. Acetone solutions were sprayed on plywood panels at the rate of 100 and 10 mg. of the toxicant per square foot. Enough panels were sprayed to avoid the necessity of using the same surface twice. In each test 20 to 40 mosquitoes under half sections of Petri dishes were exposed to the treated panels for periods ranging from 1 to 120 minutes. The insects were then transferred to cylindrical screen cages and provided with a 10% honey solution in cotton pads and mortality counts were made after 24 hours. Thirty-minute knockdown counts were also taken after each exposure up to 30 minutes. Evaluation of species susceptibility was based on a comparison of the exposure times needed to obtain 90 to 100%l mortality in 24 hours. The results are given in Table 1. Aedes aegypti was the most susceptible species to these three compounds. The ten-mg. deposits of Bayer 29493 and malathion produced 90 to 100% mortality of aegypti after exposures as short as one minute, but a 15-minute exposure was required to obtain the same result with DDT. Against taeniorhynchus, quadrimaculatus, and albimanus, Bayer 29493 was the most toxic compound and malathion was superior to DDT. Differences between the susceptibilities of these three species were minor but taeniorhynchus was slightly more susceptible than the others to Bayer 29493 and quadrimaculatus to DDT. The 100-mg. deposits of all three compounds and the ten-mg. deposits of malathion and DDT produced 10O%o knockdown of aegypti within 30 minutes following one or more of the exposures. Complete knockdown of taeniorhynchus was also obtained with the 100-mg. deposit of malathion. None